It has previously been proposed to make safety thermoelements exposed, for example, to a pilot flame or to a flame from a gas burner by assembling a pin made of constantan-metal with an outer jacket made of a nickel alloy, and then to connect the metals of the resulting junction to a safety circuit cutting off gas flow unless the assembly is exposed to a flame. To make such units, an inner conductor made of a constantan pin is manufactured; an outer conductor made of a nickel alloy is also separately manufactured and the two conductors are then connected by welding at a heat bonding junction. This method is comparatively complex and requires great care to insure that the elements at the heat bonding junction are reliably perfectly electrically connected. Cold extrusion of materials of different metals, and of different characteristics has previoulsy been proposed. For example, it is known to manufacture center electrodes for automotive spark plugs by jacketing an inner core made of copper, which has good heat conductive charactertistics, with a strong jacket made of nickel, which provides corrosion resistance.